As is known in the art, metal-insulator-metal (MIM) structures have a variety of potential applications, including as components of electrical devices, e.g., capacitors; optical devices, e.g., etalons; and micro-electrical mechanical devices (MEMs).
However, the range of potential uses of MIM structures has been constrained by the difficulty of their fabrication using prior art methods, as well as the limited types of structures that can be made using same. For example, FIGS. 1A and 1B illustrate cross-sectional views of a prior art method for fabricating MIM structures. As illustrated in FIG. 1A, a patterned mask 150 is applied to the lower surface 120 of an insulative substrate 110, e.g., using known photolithography and wet chemistry techniques. An insulator layer 155, e.g., silicon nitride (SiN), is applied to the upper surface 121 of substrate 110. The masked substrate 110 is then exposed to an appropriate etchant, such as potassium hydroxide (KOH), which removes a portion of substrate 110 so as to form cavity 130 in the back-side of substrate 110. The etching process releases a portion of insulator layer 155 from the underlying substrate 110. As illustrated in FIG. 1B, the photolithographic mask 150 may be removed, and metal layers 160, 170 applied to the exposed portion of oxide layer 155, for example using sputtering, so as to form a MIM structure.
FIGS. 2A-2B illustrate cross-sectional views of a different prior art method for fabricating a MIM structure. As illustrated in FIG. 2A, a buried sacrificial layer 220 such as SiO2 may be provided in substrate 210, and a MIM structure including insulator layer 250 sandwiched between metal layers 240, 260 may be disposed on a portion of the substrate 211 that at least partially overlies the buried sacrificial layer 220. As illustrated in FIG. 2B, substrate 210 may be exposed to an appropriate etchant, such as hydrogen fluoride (HF), or xenon difluoride (XeF2), which removes sacrificial layer 220 and defines a cavity 230. A residual portion 211 of substrate 210 overhangs cavity 230, upon which MIM structure 240, 250, 260 is disposed. These structures may be fabricated using known techniques, e.g., photolithography, wet chemistry, sputtering, and the like.
One limitation of techniques such as illustrated in FIGS. 1A-1B and 2A-2B is that the entire substrate must be exposed to chemical etchants in order to create cavities 130 or 230. As such, to avoid the risk of inadvertently etching other structures, only certain types of materials may be used in the substrate and/or one or more masks or protective layers must be provided. This can add multiple processing steps to the MIM fabrication process. Additionally, only certain types of structures may be fabricated. For example, the technique illustrated in FIGS. 2A-2B relies on the excavation of sacrificial layer 220, and as a result, MIM structure 240, 250, 260 rests on a residual portion of the substrate 211 that overhangs cavity 230 from only one side. This overhang may reduce mechanical stability, as well as potential commercial applicability of the resulting device. Moreover, the lower surface of MIM structure 240, 250, 260 is mechanically, thermally, and electrically coupled to substrate portion 211, limiting its applicability for uses that require enhanced isolation, e.g., sensors.